Method for controlling a piezoelectric actuator which is used to displace an element

ABSTRACT

The invention relates to the return stroke between an actuator and an element. In order to displace the element the actuator (A) is impinged upon by a control voltage (ANS). The control voltage (ANS) is selected in such a manner that it is dependent on the size of return stroke in order to compensate the alteration in the return stroke.

[0001] The invention relates to a method for controlling a piezoelectricactuator which is used to displace an element. With a method of thiskind the actuator has a control voltage applied to it. Because of thechange in length of the actuator induced by the activation the elementis displaced.

[0002] Piezoelectric actuators are used for example in fuel injectionsystems. In this case the element is a control valve which is operatedby the actuator.

[0003] Since the length of the piezoelectric actuator is dependent ontemperature, a safety gap, referred to as the return stroke, shouldexist between the actuator in the non-activated state and the element,said gap ensuring that even in the event of temperature changes thenon-activated actuator does not displace the element.

[0004] Efforts are being made to make the size of the return strokeindependent of ambient influences, as otherwise the element will bedisplaced more or less forcefully or in the extreme case not all whenthe actuator is activated. With a fuel injector, a change in the returnstroke effects for example a delay in the start of injection and the endof injection, which can result in high emissions and loud combustionnoises.

[0005] It is known that the influence of temperature fluctuations on thereturn stroke can be diminished if elements which determine the size ofthe return stroke are made of materials with coordinated thermalcoefficients of expansion. Invar, for example, has revealed itself to bea suitable material for a housing for the piezoelectric actuator.

[0006] However, compensating for the temperature-induced expansion ofthe actuator does not entirely succeed by this measure, since not allthe components surrounding the actuator can be made of invar. Moreover,the measure does not help in the case of changes in the return strokecaused by ambient influences that are not temperature-induced.

[0007] The object of the invention is to specify a method forcontrolling an actuator which is used to displace an element, whereinthe displacement of the element is influenced by ambient conditions to alesser extent than compared with the prior art.

[0008] The object is achieved by a method for controlling apiezoelectric actuator which is used to displace an element with thefollowing features: between the actuator and the element there is areturn stroke. In order to displace the element a control voltage isapplied to the actuator. The control voltage is selected such that it isdependent on the size of the return stroke with the aim of compensatingfor a change in the return stroke.

[0009] With this method no attempt is made to make the size of thereturn stroke independent of ambient influences; instead, a change inthe return stroke is compensated for by a suitable control voltage.

[0010] If, for example, the return stroke increases in size, then thecontrol voltage is selected correspondingly higher so that the actuatorcan overcome the greater distance to the element and displace theelement with the necessary force. Even the smallest changes in returnstroke can be compensated for by this method, which means that thedisplacement of the element is essentially independent of ambientconditions. Since by means of this method the change in return strokecan in principle be compensated for independently of its cause, themethod is also suitable for eliminating the influence of ambientconditions that are not temperature-induced on the displacement of theelement.

[0011] Preferably the method is performed in such a way that the controlvoltage is formed from a sum of at least one first voltage and acompensation voltage, with only the compensation voltage being dependenton the size of the return stroke. Because of the separation of thecompensation voltage as a summand to be added, the method can beimplemented by means of a particularly simple and straightforwarddevice. Toward that end, the device comprises on the one hand a basicvalue calculation that is permanently active and on the other hand acorrection value calculation which is performed as a function of theambient conditions.

[0012] The first voltage can be a fixed basic voltage.

[0013] In particular for use in fuel injector applications, the firstvoltage is preferably dependent on variables such as, for example,engine load, engine speed or fuel pressure so that the amount of fuel tobe injected, the start of injection and the end of injection can beadjusted to the current operating condition, such as, for example,accelerating from rest or coasting to a halt.

[0014] In order to establish the compensation voltage, the value of aparameter which serves as a measure for the current size of the returnstroke is determined.

[0015] The capacitance of the actuator, for example, can be used for theparameter. For example, the parameter can be the capacitance of theactuator. If a plurality of actuators are disposed in the same deviceand if they perform essentially the same function, the parameter caninstead be a mean value of the capacitances of the actuators. This isthe case, for example, with a multi-cylinder engine which has one fuelinjector per cylinder.

[0016] It has been shown that the capacitance of the actuator isdependent on the temperature of the actuator irrespective of how it iscontrolled. From the temperature of the actuator deductions can in turnbe made about the temperature-induced change in the return stroke, sothat the capacitance of the actuator represents a measure for the sizeof the return stroke. The use of such a parameter permits thecompensation of temperature-induced changes in the return stroke.

[0017] If the actuator and the element are used in a fuel injector, avalve is used as the element, said valve regulating the amount of fuelto be injected.

[0018] In this case the fuel temperature can be used for the parameter.

[0019] The coolant temperature can also be used for the parameter. Inthis case, during the determination of the dependence of the controlvoltage on the value of the parameter, the running time of the engine isalso taken into account in that the temperature of the actuator isdeduced from the coolant temperature at the time of starting the engineand from the current running time of the engine.

[0020] A stored assignment of parameter values to compensation voltagescan be used to determine the compensation voltage from the value of theparameter.

[0021] The assignment can be determined on the basis of a model actuatorand subsequently stored in all actuators manufactured in accordance withthe model actuator. To this end, a number of different ambientconditions of the model actuator are set. The associated value of theparameter is determined for each of the ambient conditions. For each ofthe ambient conditions, the particular control voltage at which theactuator operates the element in the desired fashion, e.g. with aspecific force, is determined. If the first voltage is known, thecompensation voltage can be determined from the control voltage. For usein a fuel injector application, the compensation voltage can beestablished by means of a check to determine at which control voltagethe right amount of fuel is injected.

[0022] The value of the parameter for a specific environmental conditioncan, however, vary from actuator to actuator for manufacturing processrelated reasons. In order to be able to adjust the initially storedassignment determined on the basis of the model actuator in simplefashion to the particular actuator, it is advantageous to determine thecompensation voltage in the method for controlling the actuator byforming the difference value from the value of the parameter and a fixedstored value and then determining the corresponding compensation voltagefrom a stored assignment of difference values to compensation voltages.This assignment can also be determined on the basis of a model actuator.In this case it is sufficient for the purposes of adjustment to measurethe value of the parameter under the same ambient condition under whichthe fixed stored value was measured in the case of the model actuator,and in the event of a deviation to replace the fixed stored value withthe measured value of the parameter of the actuator.

[0023] For use in a fuel injector application, the above-mentionedambient condition is preferably the status of the fuel injector atoperating temperature, since this ambient condition can be set easilywithout additional measurements by waiting a sufficiently long timeafter startup until there is a high probability that the operatingtemperature of the fuel injector has been reached.

[0024] The calculation of the required control voltage is performed forexample by a control unit.

[0025] The charging current with which the actuator is charged duringactivation is dependent on the control voltage and is set as a functionof resistances and capacitances in the circuit of the control unit andthe actuator. Since the charging current is dependent on the size of thecontrol voltage, the expansion of the actuator is not proportional tothe control voltage. However, the control energy with which the actuatoris charged is proportional to the expansion of the actuator. The controlenergy is yielded as the result of the multiplication of the controlvoltage by the charging current and the charging time. Consequently itis recommended that the control voltage be determined on the basis of acalculation of the control energy. Since the charging current is clearlyand invariably dependent on the control voltage and the charging time isindependent of the control voltage, there is a one-to-one relationshipbetween control voltage and control energy. Consequently the controlvoltage is determined indirectly by determination of the control energy.

[0026] Thus, the first voltage and the compensation voltage can bedetermined indirectly by determination of a first energy and acompensation energy.

[0027] The stored assignment of parameter values to compensationvoltages can consist of a stored assignment of parameter values tocompensation energies.

[0028] An exemplary embodiment of the invention is explained in moredetail below with reference to the figures.

[0029]FIG. 1 shows a piezoelectric actuator with actuator housing andelement

[0030]FIG. 2 shows a flow diagram of a method for controlling theactuator

[0031] In the exemplary embodiment there are provided four fuelinjectors which feed a combustion chamber. Each of the fuel injectorshas a piezoelectric actuator A which is used to displace an element Ewhich is implemented as a control valve. Provided between the actuatorsA in the non-activated state and the elements E there is in each case agap which is referred to as return stroke L (see FIG. 1).

[0032] An electronic control unit (ECU) which controls the actuators isprovided. In the ECU, there is stored in a characteristics map a firstassignment Z1 between fuel pressure and a first energy and hence a firstvoltage S1. The first assignment Z1 specifies the control energies andhence the control voltages ANS that are required in each case atoperating temperature and at specific fuel pressures in order to achievean optimal amount of fuel to be injected. Furthermore there is stored ina further characteristics map a second assignment Z2 of differencevalues DW to compensation energies and hence compensation voltages (seeFIG. 2).

[0033] In order to activate the actuators A, the capacitances C1, C2,C3, C4 of the actuators A are determined in a first step. From thecapacitances C1, C2, C3, C4, the mean value CM is formed. This meanvalue CM is deducted from a fixed stored value W1 in order to form adifference value DW. The fixed stored value W1 represents the mean valueof the capacitances at an operating temperature that lies between 70° C.and 90° C.

[0034] On the basis of the second assignment Z2 the associatedcompensation energy and hence compensation voltage is determined fromthe difference value DW.

[0035] The fuel pressure is measured. On the basis of the firstassignment Z1 the corresponding first energy and hence the first voltageS1 is determined from the fuel pressure.

[0036] Next, the first energy and the first compensation energy andhence the first voltage S1 and the compensation voltage AS are added inorder to determine the control energy and hence the control voltage ANSof the actuators A.

[0037] The actuators A are activated by means of this determined controlenergy or control voltage ANS in order to displace the elements E.

1. Method for controlling a piezoelectric actuator which is used todisplace an element, wherein a return stroke (L) exists between theactuator (A) and the element (E), with a control voltage (ANS) beingapplied to the actuator (A) in order to displace the element (E), withthe control voltage (ANS) being selected such that it is dependent onthe size of the return stroke (L) with the aim of compensating for achange in the return stroke (L).
 2. Method according to claim 1, whereinthe control voltage (ANS) is formed from a sum of at least a firstvoltage (S1) and a compensation voltage (AS), with only the compensationvoltage (AS) being dependent on the size of the return stroke (L), withthe value of a parameter which is used as a measure for the current sizeof the return stroke (L) being determined in order to establish thecompensation voltage (AS).
 3. Method according to claim 2., wherein thecapacitance of the actuator (A) is used for the parameter.
 4. Methodaccording to one of claims 1 to 3, wherein the actuator (A) and theelement (E) are used in a fuel injector, with a valve which regulatesthe amount of fuel to be injected being used as the element (E). 5.Method according to claim 2 to 4, wherein the fuel temperature is usedfor the parameter.
 6. Method according to claim 2 to 4, wherein thecoolant temperature is used for the parameter, with an engine beingdriven by means of the fuel injector with the running time of the enginebeing taken into account in determining the dependence of thecompensation voltage (AS) on the value of the parameter.
 7. Methodaccording to one of claims 2 to 6, wherein the compensation voltage (AS)is determined in that the difference value (DW) is formed from the valueof the parameter and a fixed stored value (W1) and then thecorresponding compensation voltage (AS) is determined from a storedassignment (Z2) of difference values (DW) to compensation voltages (AS).8. Method according to claim 7 and one of claims 2 to 6, wherein at atime after startup of the fuel injector at which there is a highprobability that the fuel injector has reached its operating temperaturethe value of the parameter is determined, compared with the fixed storedvalue (W1) and in the event of a deviation replaces the fixed storedvalue (W1).
 9. Method according to one of claims 1 to 8, wherein thecontrol voltage is selected indirectly by determination of the controlenergy.